A condenser microphone comprises a condenser microphone unit having a very high impedance. Accordingly, the condenser microphone contains an impedance converter such as an FET (field effect transistor). Typically, a phantom power source is used in the condenser microphone. A microphone output is provided via a balanced shield cable of the phantom power source.
For example, a hand (hand-held) microphone normally comprises a 3-pin type output connector. The microphone can be attached to and detached from a microphone cable. The output connector is specified in EIAJ RC-5236 “Latch Lock Type Round Connector for Acoustic Equipment”. By way of example, as shown in Patent Document 1 (Japanese Patent Application Publication No. H11-341583), a cylindrical base consisting of an electric insulator is penetrated by three pins, a first pin for grounding, a second pin used as a hot side of a signal, and a third pin used as a cold side of a signal.
A microphone case is a shield case consisting of a metal material, for example, brass. The microphone case contains an electronic circuit comprising an output circuit including an impedance converter for the condenser microphone unit and a polarization voltage generating circuit, or the like. However, when an intense electromagnetic wave is applied to the microphone or microphone cable, it may enter the microphone case via the output connector from the microphone cable. The electromagnetic wave is then demodulated by the impedance converter. As a result, noise of an audible frequency may be output from the microphone.
As a technique for preventing noise resulting from the entry of an electromagnetic wave from the output connector into the microphone case, a method has been proposed which connects the ground of the electronic circuit to the microphone case by wiring and which then couples the grounding first pin, included in the output connector, directly to the microphone case (see Non-Patent Document 1; Jim Brown and David Josephson “Radio Frequency Susceptibility of Capacitor Microphones” Audio Engineering Society Convention Paper 5720 (p. 12, FIG. 8)).
The method described in Non-Patent Document 1 prevents a loop current path from being formed by the floating capacity between the electronic circuit and the microphone case. Further, this method does not involve wiring joining the grounding first pin and the ground of the electronic circuit together, that is, it does not involve anything acting as an antenna. This makes it possible to effectively prevent the entry of an electromagnetic wave.
However, according to the method described in Non-Patent Document 1, the grounding first pin is coupled directly to the microphone case, so that the use of the phantom power source results in the passage of a current through the microphone case. Consequently, if the grounding first pin gets detached from the microphone case for any reason, the potential of the microphone case may increase to 30 V or higher in the case of a 48-V phantom power source. In this case, touching the microphone case with the hand may result in an electric shock.